Anti-static coating for photographic emulsion layers

ABSTRACT

This application describes a method of improving the surface electrical conductivity of photographic material which is characterized in that it comprises coating on to the surface of the photographic material an aqueous solution of kappacarrageenan and a sufficient amount of a salt of potassium or ammonium to cause the kappa-carrageenan to gel, allowing the coated layer to gel and drying the photographic material.

United States Patent Wood [151 3,655,384 5] Apr.1l, 1972 [54]ANTI-STATIC COATING FOR PHOTOGRAPHIC EMULSION LAYERS [72] Inventor:Henry Walter Wood, Ilford, England [73] Assignee: Ilford Limited,llford, Essex, England 22 Filed: July 31, 1970 [21] Appl. No.: 60,100

[52] US. Cl. ..96/67, 96/87 A, 96/114.2,

117/201 [51] Int. Cl ..G03c H76 [58] Field ofSear-ch ......96/1 14.2, 87A; 117/201 [56] References Cited UNITED STATES PATENTS 2,772,165 11/1956Moede ..96/114 2,863,769 12/1958 Moede ..96/1 14 3,184,311 5/1965Salminen ..96/87 A 3,250,620 5/1966 Lovett Primary Examiner-Nonnan G.Torchin Assistant Examiner-J. R. Hightower Attorney-Cushman, Darby &Cushman [57] ABSTRACT 6 Claims, No Drawings ANTI-STATIC COATING FORPHOTOGRAPHIC EMULSION LAYERS This invention relates to colloid silverhalide photographic film material provided with an antistatic layer andto the production of such film material.

It is known that photographic film material exhibits a tendency to pickup electrostatic charges during the course of its manufacture and alsoduring its use. When an electrostatically charged film is discharged socalled static marks are produced. These marks show as areas of localover-exposure on the developed film. It is possible to reduce thetendency of photographic film to pick up electrostatic charges by theprovision of a layer which has a relatively high electroconductivity.Any electrostatic charge picked up by a film having such a layer rapidlyleaks to earth without causing a discharge.

It is the object of the present invention to provide a method ofimproving the surface conductivity of photographic material.

According to the present invention there is provided a method ofimproving the surface electrical conductivity of photographic filmmaterial which comprises coating on to a surface of the photographicmaterial an aqueous solution of kappa-carrageenan and a sufficientamount of a salt of potassium or ammonium to cause the kappa-carrageenanto gel, allowing the coated layer to gel and then drying thephotographic material.

Carrageenan is a natural gum derived from seaweed. It usually exists asa mixture of kappaand lambda-carrageenan sometimes together with minorproportions of other carrageenans. Carrageenan is a polygalactose andthe difference between the kappa and lambda varieties lies in structuraldifferences in the complex chains. However kappaand lambdacarrageenancan readily be distingrished by the fact that kappa-carrageenan isprecipitated from an aqueous solution by potassium salts whilelambda-carrageenan is not so precipitated. However in the process of thepresent invention the kappa-carrageenan used need not be purekappa-carrageenan and a proportion of lambda-carrageenan and possiblyminor amounts of other carrageenans may also be present. Therefore theterm kappa-carrageenan as used herein refers to carrageenan whichcomprises from 70 to 100 percent kappa-carrageenan.

For the same strength of solution in water kappa-carrageenan solutionsare more viscous than gelatin solutions and thus aqueous solutions ofkappa-carrageenan stronger than 1 percent by weight tend to give rise todifficulty in coating. On the other hand an aqueous solution ofkappa-carrageenan which is weaker than 0.2 percent by weight tends to bedifficult to gel. Thus the preferred concentration of kappa-carrageenanfor use in the present invention is 0.2 to 1 percent by weight andpreferably 0.4 to 0.6 percent by weight.

The minimum amount of a salt of potassium or ammonium required to causethe kappa-carrageenan to gel is approximately 0.05 percent in theaqueous coating solution. However a too strong concentration of the saltin the coating solution causes precipitation of the carrageenan (ashereinbefore mentioned). In such cases, on drying, a layer is formedwhich has a relatively low conductivity. For example 0.5 percent byweight of potassium chloride in an aqueous kappa-carrageenan solutioncauses precipitation of the kappa-carrageenan.

Furthermore the higher the concentration of potassium or ammonium saltpresent the higher the setting temperature of the gel. in every case thecoating solution should be such that it may be coated at a temperaturebelow the melting point of the colloid of the photographic emulsionwhile itself remaining ungelled. Thus the preferred concentration of asalt of potassium or ammonium in the coating solution used in theprocess of the present invention is from 0.05 to 0.2 percent by weight.

The present invention includes photographic material which comprises asupport base having on one side a colloid silver halide layer and on theother side a layer of gelled kappa-carrageenan, or photographic materialwherein the layer of gelled kappa-carrageenan is on the same side as thecolloid silver halide layer either coated on a supercoat layer on thecolloid silver halide layer or preferably, coated directly as asupercoat layer on the colloid silver halide layer. Preferably, in thecase of X-ray film material which comprises a colloid silver halidelayer on each side of the support base there is present a gelledkappa-carrageenan layer on each colloid silver halide layer as thesupercoat layer.

The following example will serve to illustrate the invention.

EXAMPLE Strips of X-ray film emulsion coated on each side of a basesupport were coated on each emulsion layer by the process of the presentinvention. There was formed a supercoat layer on each side of thephotographic material which comprised a gelled layer ofkappa-carrageenan. The actual coating solutions used in each case areshown in the table which follows. These strips were compared to asimilar strip of X-ray film material which had on each side a gelatinsupercoat.

Two tests were carried out on each of the strips. In one test the filmstrips were brought to equilibrium at a relative humidity of 20 percentand their resistivities at this humidity were measured under an appliedpotential of l0kV. The results of this test are shown in the table asset forth below under the heading Log R.

In another test, also carried out at 20 percent R.H. the propensity ofthe film strips to static formation was assessed. Strips were charged bybeing passed between rubber rollers to create a static charge which wasmeasured by a juxtaposed field meter. An indication of how quickly theapplied charge was dissipated was obtained by measuring the field againafter 20 seconds. The results are shown also in the table which follows.

The kappa-carrageenan used in the foregoing example was supplied byMarine Colloids lnc., and comprises 83 percent kappa and 17 percentlambda forms.

Reduction of propensity to static and decreased resistivity is thus inevidence with all the mixtures of salt and carrageenan. The potassiumsalt however, is especially efficacious, and at the 0.2 percent levelhas substantially inhibited formation of static.

The new supercoats have substantially no effect on the initialphotographic properties of the emulsion.

I claim as my invention:

1. A method of improving the surface electrical conductivity ofphotographic material comprising a film base and at least one silverhalide emulsion layer, said method comprising the steps of: coating onto the surface of the photographic material an aqueous solution ofkappa-carrageenan and at least 0.05 percent by weight, based on theweight of said solution, of a chloride of potassium or ammonium, theamount of said chloride being sufficient to cause the kappa-carrageenanto gel, allowing the coated layer to gel and drying the photographicmaterial so as to provide thereon an antistatic coating consistingessentially of the gelled kappa-carrageenan and said chloride.

5. A method according to claim 1 wherein the photographic materialcomprises a film base carrying on one side only thereof at least onephotographic silver halide emulsion layer and the said aqueous solutionis applied to the other side of the film base.

6. Photographic material comprising a film base, at least one silverhalide emulsion layer and a dried layer of a gelled kappa-carrageenanand potassium or ammonium chloride.

2. A method according to claim 1 wherein concentration ofkappa-carrageenan in the aqueous solution is from 0.2 to 1 percent byweight of said solution.
 3. A method according to claim 1 wherein theconcentration of chloride of potassium or ammonium in the coatingsolution is from 0.05 to 0.2 percent by weight.
 4. A method according toclaim 1 wherein the photographic material comprises a film base carryingat least one photographic silver halide emulsion layer and the saidaqueous solution is applied to the surface of the outermost emulsionlayer.
 5. A method according to claim 1 wherein the photographicmaterial comprises a film base carrying on one side only thereof atleast one photographic silver halide emulsion layer and the said aqueoussolution is applied to the other side of the film base.
 6. Photographicmaterial comprising a film base, at least one silver halide emulsionlayer and a dried layer of a gelled kappa-carrageenan and potassium orammonium chloride.